Lead free, tin-bismuth solder alloys

Abstract: Disclosed is a lead free alloy. The alloy is a low solidus temperature, multi-component solder alloy containing at least about 50 weight percent Bi, up to about 50 weight percent Sn (basis total Sn and Bi), and an effective amount of a physical and mechanical property enhancing third component. The third component can be Cu, In, Ag, and combinations of Cu and Ag.

Description: The invention relates to lead free solders and especially to lead free, tin-bismuth solder alloys. These alloys are low toxicity solder alloys that are particularly useful in microelectronic applications. The solder alloys of the invention are particularly useful in joining integrated circuit chips to chip carriers and substrates, as printed circuit boards, joining chip carriers to substrates, and joining circuitization lands and pads in multilayer printed circuit boards. Background of the Invention Soldering is a low temperature, generally reversible, metallurgical joining process. Low temperature and reversibility are especially important in electronics applications because of the materials involved and the necessity for reworking and making engineering changes. Solder joining is a wetting process followed by a chemical reaction. Molten solder wets selectively. The selective wettability of solders allow molten solder to be confined to desired sites. This is especially important in flip chip bonding, and in working with solder masks. The soldering process can be accomplished as quickly as the wetting takes place, for example, on the order of a few seconds. This makes soldering particularly desirable for automated, high speed, high throughput processes. Wettability is also a function of the materials to be joined, with Cu, Ni, Au, and Pd, as well as alloys rich in one or more of these metals, being particularly amenable to soldering. The chemical reaction following wetting is between the molten solder and the joining metallurgy to form an intermetallic phase region at the interface. The intermetallic phases formed by solders in electronic packaging are stoichiometric compounds, typically binary compounds, and typically containing Sn if Sn is present in the solder alloy. When the base, pad, or land is Cu, and the solder alloy is rich is Sn, the intermetallic formed during soldering is Cu-Sn. Exemplary Cu-Sn binaries include Cu 3 Sn and Cu 6 Sn 5 . Solder alloys are characterized by the melting temperature being a strong function of composition. While a pure metal is characterized by a single, invariant, melting temperature, the freezing and melting points of alloys are complex. The freezing point of an alloy is determined by the liquidus line. Above the liquidus line only a liquid phase or phases can exist. The melting point of an alloy is determined by the solidus line. Below the solidus line only a solid phase or phases can exist. In the region between these two lines, i.e., between the liquidus line and the solidus line, solid and liquid phases can co-exist. The preferred soldering alloys are eutectics, that is, they are characterized by a eutectic point. The eutectic point is where the liquidus and solids lines meet. A concentration change in either direction from the eutectic results in an increase in the liquidus temperature. The composition, and the quench rate, also determine the microstructure and the resulting mechanical properties of the solder joint. Thus, it is necessary to both carefully choose the solder composition and to carefully control the thermal exposure of the soldered joint. A solder composition used in electronics fabrication must be wettable as a solder alloy, and have at least one component capable of forming an electrically conductive, thermally stable, non-brittle, plastic intermetallic with the pad or land metallurgy. For this reason, the most common solder alloys are lead based alloys, as Sn-Pb alloys. Heretofore, Pb/Sn solders have been utilized for electronic applications. There have been many historical reasons for the wide spread use of Pb/Sn alloys. These historical reasons include the low solidus temperature of Pb/Sn solder alloys, the workability of Pb/Sn alloys and of the resulting Cu/Sn intermetallics (formed at the solder/Cu contact interface) over a wide temperature range, the adhesion of Cu/Sn intermetallics obtained from Pb/Sn alloys to Cu lands and pads, and the ready availability of process equipment and low cost adjuncts, as resins, fluxes, and solder masks, for Pb/Sn alloys. The relatively low temperatures required for processing Pb/Sn solder alloys are particularly important when polymeric dielectrics are used in the fabrication of electronic packages. These polymers can degrade in high temperature assembly operations. Solder alloys which melt at relatively low temperatures can accommodate these polymeric substrates. Additionally, semiconductor chips are subject to thermal diffusion and structural transformations at elevated temperatures. Low melting solders avoid these problems. Especially important is the "softness" or plasticity of lead based solders. This softness or plasticity allows the solder to accommodate the mismatch in coefficients of thermal expansion between the bonded structures, for example the mismatch in coefficient of thermal of thermal expansion between a ceramic dielectric and a polymeric dielectric, or between a semiconductor chip and a ceramic or polymeric chip carrier or substrate. However, lead is a toxic, heavy metal with a relatively high vapor pressure. Its use is disfavored, and a need exists for a replacement. Exemplary lead-free alloys are described, for example in EP 0-354-570-A2 to A.J.G. Strandjord, R.L. Yates, and D.J. Perettie for OPTICAL INFORMATION STORAGE MEDIUM which describes an optical data storage media containing an alloy of 40-85% Sn, 10-50% Bi, 1-15% Cu. Lead free solder alloys are disclosed in, for example USP 4,929,423 to K. L. Tucker and U. Ma for LOW TOXICITY ALLOY COMPOSITIONS FOR JOINING AND SEALING which describes a lead free solder alloy for joining and sealing, disclosed to be useful as a plumbing alloy. The disclosed alloy contains 0.08-20% Bi, Cu, Ag, P, Rare Earth, and balance (ca. 80%) Sn Seraphim, Lasky & Li, PRINCIPLES OF ELECTRONIC PACKAGING , shows, at Table 19.1, a lead free alloy containing 58% Bi and 42% Sn. GB-A-1468973 relates to a method of soldering a lid on the substrate of a printed circuit to form a closed micro-circuit package. Suitable solder alloys for soldering the lid are also disclosed. In Chemical Abstracts, Vol. 86, no. 10 (07.03.77), abstract no. 59289 (corresponding to JP-A-51108625) there is disclosed a lead free solder alloy, comprising 36-54% Bi, 44-52% Sn and 2-12% In. However, the above alloys fail to provide a lead free solder that flows at low enough temperatures to avoid damage to electronic materials, wets the bonding metallurgy typically used in electronics fabrication, e.g., Cu, Au, Ag, and Pd, especially while avoiding wetting organic materials as substrates and solder masks. Objects of The Invention It is a primary object of this invention to provide a lead free solder. It is a further object of this invention to provide a lead free solder that wets and forms a chemically and thermally stable intermediate the bonding metallurgy typically used in electronics fabrication, e.g., Cu, Au, Ag, and Pd, especially while avoiding wetting organic materials as substrates and solder masks. It is a further object of this invention to provide a lead free solder having that flows at low enough temperatures to avoid damage to electronic materials. SUMMARY OF THE INVENTION These and other problems of prior art solder compositions are overcome by the lead free solders of the present invention. The solder compositions disclosed herein are low solidus temperature, multi-component solder alloys. The principal metals in the alloy are Bi and Sn. In a first embodiment of the invention, the low solidus temperature, multi-component solder alloy consists of 56 weight-% Bi, and 42 weight-% Sn, and 2 % weight-% of a physical and mechanical property enhancing third component, namely In. In an alternative embodiment of the invention the alloy contains 46 weight-% Bi, 48 weight-% Sn, and an effective amount of a physical and mechanical property enhancing third component,namely a combination of 4 weight-% Cu and 2 weight-% Ag. Detailed Description of the Invention According to the invention there is provided a family of lead-free, Sn-Bi solders. These solders are characterized by a low solidus temperature. The principal metals in the alloy are Bi and Sn. The low solidus temperature, multi-component solder alloy has major portions of Bi, and Sn, and an effective amount of a physical and mechanical property enhancing third component. The physical and mechanical property enhancing third component may be In, or a combination of Cu and Ag. In the particular embodiment of the invention, the solidus lowering third component is a mixture of Ag and Cu. A Bi-Sn-Cu-Ag alloy composition contains 46 weight percent Bi, 48 weight percent Sn, (i.e., at least about 50 weight percent Sn, and less then 50 weight % Bi, basis total Bi and Sn), 4 weight percent Cu and 2 weight percent Ag. In an alternative embodiment, the solidus lowering third component is In. A preferred Bi-Sn alloy is one containing 56 weight percent Bi, 42 weight percent Sn, and 2 weight percent In. The following method of electrically connecting an integrated circuit chip to a circuitized substrate does not form part of the present invention, but is given as an example for applying the multicomponent solder alloy. This interconnection method includes the step of depositing a solder alloy comprising Bi, Sn, and an effective amount of a physical and mechanical property enhancing third component. This alloy is deposited onto electrical contacts of the integrated circuit chip. The solder alloy may be applied by wave solder deposition, electrodeposition, or as a solder paste. The electrical leads of the circuitized substrate are then brought into contact with the solder alloy on the electrical contacts of the integrated circuit chip. Where the chip is to be mounted in a "flip chip" conformation, the current leads of the circuitized substrate are pads on the substrate, and the solder alloy deposits are brought into contact with the pads. Alternatively, where the integrated circuit chip is mounted right side up, the current leads are wire leads, and tab inner lead connections, and they are brought into contact with the solder alloy contacts on the top surface of the integrated circuit chip. While the substrate current leads and the solder deposits are maintained in contact the solder alloy is heated to cause the solder alloy to wet and bond to the electrical leads of the circuitized substrate. Heating may be by vapor phase reflow, infrared reflow, laser reflow, or the like. The resulting microelectronic circuit package of the invention is an integrated circuit chip module with a circuitized chip carrier, i.e., a substrate, a semiconductor integrated circuit chip, and an alloy formed of major portions of Bi, and Sn, and an effective amount of a physical and mechanical property enhancing third component, which may be In, or the combination of Cu with Ag, as an electrical solder bond interconnection between the circuitized chip carrier and the semiconductor integrated circuit chip.

Claims: A low solidus temperature, multicomponent solder alloy consisting of 56 weight-% Bi, 42 weight-% Sn, and 2 weight-% In. A low solidus tempearuture, multicomponent solder alloy consisting of 46 weight-% Bi, 48 weight-% Sn, 4 weight-% Cu and 2 weight-% Ag.

Inventors: GONYA STEPHEN GILBERT, LAKE JAMES KENNETH, LONG RANDY CLINTON, WILD ROGER NEIL, GONYA, STEPHEN GILBERT, LAKE, JAMES KENNETH, LONG, RANDY CLINTON, WILD, ROGER NEIL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1994
Main classification: B23K35
IPC codes: H05K3, B23K35, H01L21, C22C30, C22C13, C22C12, C22C28